1. Field of the Invention
This invention relates to sensors, including temperature sensors.
2. Introduction to the Invention
A wide variety of electronic components and other articles are subject to damage if exposed to elevated temperature. It is, therefore, often important to be able to determine if a component has been subjected to such temperature. Various detection techniques, e.g. thermochromic materials which change color when exposed to a specific temperature, have been proposed for this purpose. Such techniques suffer from the requirement that the article must be visible in order to detect the color change, and thus are ineffective when the article is enclosed. Various electronic detectors, designed to identify an electrical change resulting from a high temperature, have also been proposed. Such detectors may not be able to determine whether a particular part of an article has been exposed to a high temperature, relying instead on the average over the entire surface. In addition, sensors which are able to maintain direct contact with the substrate, even when the substrate is not flat, are desirable. Such sensors would have sufficient flexibility that they could provide two-dimensional sensing over a large surface, and be able to be bent over an edge to provide three-dimensional sensing.
Conductive polymer compositions exhibiting a positive temperature coefficient of resistance (PTC) effect are well known. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler. At low temperatures the composition has a relatively low resistivity. However, when the composition is exposed to a high temperature, due for example, to a high current condition, the resistivity of the composition increases, or "switches", often by several orders of magnitude. The temperature at which this transition from low resistivity to high resistivity occurs is the switching temperature, T.sub.S. T.sub.S is defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of an element prepared from the composition as a function of temperature which lie on either side of the portion of the curve showing a sharp change in slope.
The use of a sensor comprising a PTC conductive polymer to detect an overtemperature condition is known. For example, Japanese Patent Application No. 10-95019, filed Apr. 7, 1998 (K. K. Raychem), the disclosure of which is incorporated herein by reference, discloses a elongate temperature sensor which can be used to detect overheating in a battery. Batteries which overheat are subject to damage, and in addition may damage the packaging surrounding them and the components in contact with them. While overheating may be due to external environmental conditions, for secondary, i.e. rechargeable batteries, such overheating may occur as a result of excessive charging. The overheating may result in damage to the internal components of the battery, the generation of gas, and, under extreme conditions, explosion of the battery. For example, for nickel-metal hydride batteries, it is desirable to keep the temperature below 100.degree. C. to avoid the. evolution of hydrogen. It is, therefore, important to identify batteries which have been subject to overheating before damage can occur. In Japanese Patent Application No. 10-95019, a sensor is attached to a plurality of batteries. An elongate tape composed of a PTC conductive polymer comprising spaced-apart sensing components and connecting components is in contact with the individual battery cells. The sensing components are electrically connected in series so that the resistance of the sensor is the sum of the resistances of each individual sensing component. The sensor is positioned so that a sensing component is in contact with the external surface of a battery cell, and preferably each individual battery cell contacts a different sensing component. When the battery cells are in a normal, low temperature condition, the resistance of the sensor is low. If, however, one battery cell heats to a temperature above T.sub.S, the resistance of the sensing component in contact with that battery cell increases, thus increasing the total resistance of the sensor and indicating that at least one battery has been subject to overheating.